Three-dimensional structures with micron-scale features have many potential applications, including photonic band gap materials, tissue engineering scaffolds, biosensors, and drug delivery systems. Consequently, several assembly techniques for fabricating complex three-dimensional structures with features smaller than 100 microns have been developed, such as microfabrication, holographic lithography, two-photon polymerization and colloidal self-assembly. However, all these techniques have limitations that reduce their utility.
Two-photon polymerization is capable of creating three-dimensional structures with sub-micron features, but from precursors that are not biocompatible and at a very slow rate, ca.<10 μm/s. Many techniques have been developed to fabricate three-dimensional photonic crystals, but they rely on expensive, complicated equipment or time-consuming procedures. Colloidal self-assembly has also been utilized to make three-dimensional periodic structures, but controlling the formation of defects is difficult.
Polymeric solutions are used in nature to fabricate thin filaments. Spiders, for example, derive their silk fibers from a concentrated protein biopolymer solution that solidifies as it is drawn to form an extremely strong filament. The extensional flow of the solution aligns liquid crystal sheets in the polymer, and the solution gels by adding ions as it leaves the spinneret. This process may be artificially recreated by the deposition of the recombinant spider silk biopolymer into a polar “deposition bath” to produce filament fibers with comparable properties.
Hydrogels are an important class of soft materials that can be fabricated in the form of three-dimensional (3D) microperiodic structures by colloidal templating or interference lithography. However, neither approach allows one to omnidirectionally vary the spacing between patterned features over length scales ranging from sub-micrometer to tens of micrometers.
The ability to pattern soft materials at the microscale is critical for several emerging technologies, including tissue-engineering scaffolds, photonic crystals, sensors, and self-healing materials. Tissue-engineering scaffolds are typically composed of synthetic or natural polymers and provide a means for tissue to grow outside the body. The scaffolds may be biodegradable or non-biodegradable. A challenge in the field is to select or design a scaffold material for cell growth and differentiation that has desirable properties, including mechanical strength, durability, and biocompatibility, and which does not induce an immune response.